Which is not a property of solids?

Options

A. Solids are always crystalline in nature
B. Solids have high density and less compressibility
C. Solids diffuse very slowly
D. The volume of solids is fixed

Answer

To determine which is not a property of solids, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Option 1: Solids are always crystalline in nature.**
- This statement is incorrect because solids can exist in two forms: crystalline and amorphous. Crystalline solids have a well-ordered structure, while amorphous solids do not have a long-range order. Therefore, this option is indeed not a property of solids.

2. **Option 2: Solids have high density and less compressibility.**
- This statement is correct. Solids typically have a high density compared to liquids and gases, and they are less compressible because their particles are closely packed together.

3. **Option 3: Solids diffuse very slowly.**
- This statement is also correct. The diffusion rate in solids is extremely low due to the tightly packed particles, making it almost negligible.

4. **Option 4: The volume of solids is fixed.**
- This statement is correct as well. Solids have a definite volume, which does not change under normal conditions.

### Conclusion:
From the analysis above, the option that is not a property of solids is **Option 1: Solids are always crystalline in nature.**